I messed around with chevy small blocks for many years. When I had my 95 gas wagon I spent a lot of time looking over all the options available for the Hummer. The engine that you have is a strong 4 bolt main marine engine that will have no problem putting out another 100 hp. The stock truck already has a very open non-restrictive exhaust system. It also has a very large air filter/cleaner so restriction is not an issue. The stock engine also has a high energy electronic ignition system with a computer controlled advance. Adding a new coil etc won't get you anything. Advancing the engine by turning the distributor won't do you any good either. If you want to advance the distributor get one of the performance chips.

Almost all of the real options will gain you more hp but in a gas engine it will be a high RPM's. In the Hummer you want the hp and torque at low RPM's. It's high RPM's that really wear engines because of the tremendously increased forces caused by the rotational and reciprocating parts. An engine is a pump. The more gas and air you can get in and out the more power. The higher the RPM's the greater the velocity of air/gas mixture entering and exiting the engine the more power. The methods used in doing the above involve changing cams, carburetors, headers etc. I think the vortec's hp occurs at a higher rpm also. It wouldn't cost that much to stick new cam in and change the chip to get more power, but it would be at higher rpm's.

It boils down to the old saying that there is no free lunch. The only way to get what you want IMHO is to either have a small block engine built for you with a stroker crank or get a 502 which has tons of low end torque because of it's stroke.

What I determined to be the best solution for a gas Hummer that would let you have your cake and eat it too is a supercharger, the Whipple unit in particular. Why? It really works. It will give you another 80 hp at the low end where you need it and it passes emissions. The kit for the Hummer is pretty good. You don't have to pull your engine or change the exhaust system or otherwise mess up the truck. It replaces the intake manifold and comes with a k&n air filter setup. You bolt your existing throttle body to the supercharger and bolt on a pulley. I drove a truck with one of these and it was everything a Hummer should be. I think it went from 0 to 60 in 13 seconds. It drove like a car. The owner said that the gas mileage was about the same. This was probably due to the increased volumetric efficiency of the new setup. The down side is that you have to burn premium, and the unit costs around $5000 un-installed. Figure another $1600 for installation. Now you know why the option for a custom new engine is not so far fetched.

I really liked my 95 gas wagon and had no problems with it. After going through the above exercise and trying to find a safe solution to adding an aux gas tank I traded my truck in for a 96 turbo diesel. If I had a gas truck with high miles or bought one used at a good price I might consider going with a new engine or supercharger.